1. Field of the Invention
The present invention relates to a method for preparing a biaxially stretched polyether ether ketone film. More specifically, it relates to a method for preparing a biaxially stretched polyether ether ketone film which inhibits the occurrence of breakages at the time of stretching, is excellent in thickness precision, and has a high dielectric breakdown voltage, no pinholes and a low shrinkage ratio by heating.
2. Description of the Related Art
A polyether ether ketone (hereinafter referred to as "PEEK") belongs to a crystalline thermoplastic resin series having the most excellent heat resistance, and it is an industrial material which is extremely excellent in chemical resistance, hot water resistance and slip properties in addition to heat resistance.
Heretofore, a polyethylene terephthalate and the like have been used as insulating materials for condensers and the like. In recent years, the demand for the condensers which can be used under specific circumstances, particularly at high temperatures has been increased. PEEK has been noted as an insulating materials for condensers which are used at high temperatures, because it can be biaxially stretched to form a thin film and is excellent in heat resistance. Additionally, PEEK is ready to be widely used as an insulating material in the fields of electrical industry, electronic industry and atomic industry.
Japanese Patent Laid-open No. 58-63417 discloses a method for preparing a biaxially oriented isotropic PEEK film in which the first axial stretching is carried out at 140.degree.-180.degree. C. in a stretching ratio of from 150% to 400%, and the second axial stretching is done at 150.degree.-200.degree. C. so that the difference between and the sum of the orientation coefficients of the first and the second stretching directions may be 0.1 or less and 0.7 or more, respectively, and heat setting is achieved, while controlled shrinkage of 20% or less is effected at a temperature of 200.degree. C. or more and the melting point or less.
Japanese Patent Laid-open No. 01-101335 discloses a biaxially oriented PEEK film which can be obtained by dispersing spherical silica particles, carrying out the first stretching at an stretching temperature (Ti) of from [glass transition temperature (Tg)-10].degree. C. to (Tg+45).degree. C. in a stretching ratio of 150% or more and the second stretching at an stretching temperature (Ti+ 15).degree. C. to (Ti+40).degree. C., and then performing heat setting at 200.degree.-350.degree. C.
Furthermore, in Japanese Patent Laid-open No. 01-283127, a method for preparing a PEEK film is disclosed in which the first and the second axial stretchings are carried out at an stretching temperature of 130.degree.-250.degree. C. in a stretching ratio of from 200% to 500%, and then heat setting is conducted in a temperature range of from 250.degree. C. to the melting point.
However, any preparation method disclosed in these publications uses high temperatures for the first stretching, and therefore crystallization takes place at the time of the first stretching, with the result that pinholes tend to occur in the film in the second stretching step, and the film breaks in a certain case. Such a film does not have a practical dielectric breakdown voltage and so it cannot be utilized as an insulating material.
Particularly, when roll stretching is carried out at the temperatures disclosed in the above-mentioned publications with the intention of obtaining a thin film having a thickness of 15 .mu.m or less, the position where the film is released from stretching rolls falls off forward from a tangential direction between the two rolls, since the adhesive properties of the film to the orientation rolls are too strong. For this reason, it is difficult to accomplish the uniform stretching. As a result, the unevenness of the stretching increases, so that a film having good thickness precision cannot be obtained. This tendency is liable to occur when a gap between the stretchings (a space between stretching rolls) is small.
Moreover, the heat setting disclosed in the above-mentioned publications is not considered to be satisfactory to obtain a film having a low shrinkage ratio by heating.